By Gary Zimmer
Rule number four of the Six Rules of Biological Farming is about cover crops: Create maximum plant diversity by using green manure crops and tight rotations. I never miss an opportunity to have something growing on my land. Green plants feed soil life, build organic matter, and capture nutrients in their tissues. Keeping my nutrients in a biological cycle means those nutrients will not leach or erode, and they are in a form that is linked to biology so it is easier for plants to access them. Nutrients held in a cover crop do not show up on a soil test, but as those plants break down, the nutrients in them are released into the soil in a plant-available form. When I do not have a forage crop or row crop growing on my land, I want to have a cover crop growing.
A good cover crop can provide many benefits, including:
- improving water infiltration into the soil
- reducing water loss from bare soil by evaporation
- holding soil in place and reduces erosion from wind and rain
- reducing fertilizer inputs by providing calcium, phosphorus, potassium, nitrogen and micronutrients to the following crop
- breaking up soil compaction producing compounds that deter weeds and crop pests
- increasing soil organic matter levels
- feeding soil biology
There are many different types of cover crops, and each provides different benefits, but there are a couple of things that all cover crops have in common: cover crops increase plant diversity in your rotation, and they pull up and hold onto soil nutrients.
The Benefits of Plant Diversity
There is a lot to be gained by adding more types of plants to your rotation through growing cover crops. Many pests, for example, prey on a relatively narrow range of species, so increasing diversity can break pest and disease cycles. Growing a wide range of plant species can also increase microbe diversity because different microbes prefer different types of plants. Another benefit of increasing plant diversity is that different types of plants access nutrients other than those the crop will pull up, so planting cover crops and working them into the soil can increase the amount and variety of plant-available nutrients. Finally, cover crops can have a different type of root system from the main crop, which will help keep channels in the soil open to allow water infiltration and air movement.
I run a lot of different test plots on my farm, and when I was first starting out I wanted to demonstrate the value of planting a diversity of plants by establishing a continuous corn plot on my farm. This sounds counterintuitive, but what made this corn-on-corn field different was that it always had one or more cover crops on it.
A lot of farmers have learned from experience that problems with diseases and pests occur when you grow corn-on-corn and don’t rotate your crops. By interseeding my corn crop with clover and planting rye each fall, I wanted to show farmers that you do not have to rotate your crops to get diversity in the system — you can get diversity by adding cover crops. Planting clover and rye with my corn meant I was growing not one crop, but three. In addition, not using herbicides gave me more plant diversity in the form of weeds. Having weeds on my fields is not all bad, as long as I use management techniques like early cultivation to keep them under control.
After ten years of planting this corn/cover crop system on the same field, a University of Illinois researcher visited my farm and took a look at the field. He was very surprised by the health of the plants and the lack of crop pests. He had never seen a 10-year continuous corn field without corn rootworms in it. He could not understand how I was able to maintain such a healthy cornfield without rotating the crop. Of course, it was not really a 10-year continuous corn field, it was a corn/clover/rye field. The diversity added by the cover crops helped break pest cycles, kept the nutrients cycling, fed soil life, and improved soil structure. By adding cover crops, corn-on-corn really can be a sustainable farming system.
Increasing plant diversity results in a wider variety of soil life and insects, and as a result no one disease or crop pest can take over. If you are planting a corn/beans rotation with no variation and no cover crops, you have neither a diversity of residues nor soil life, and as a result you will not be able to stop the diseases and insects. That is one reason there are so many bioengineered crops and such heavy pesticide use: we have removed diversity from our farming system, and as a result we are in a constant battle against insects and diseases.
Plant diversity is the key that can break those pest and disease cycles. Since each type of plant uses different minerals, cover crops put different minerals back in the ground as they break down and this feeds a variety of soil life, improving the entire biological system. This means that the more plant diversity I have, the more success I have on my farm.
Nutrients Provided by a Cover Crop
Cover crops help make nutrients more accessible to your crop by pulling them out of the soil and holding them in their tissues. When the cover crop is worked back into the soil, microorganisms digest the plant material. As those bacteria, fungi, and other soil creatures die and decay, or are consumed by other soil organisms, they release nutrients into the soil in a plant-available form. They also produce proteins and increase plant-available nitrogen through their own biological processes. This means that as a cover crop breaks down in the soil, there is an increase in soil biological activity and a release of plant-available nutrients for the next crop growing on the land.
I wanted to demonstrate just how many nutrients are held in a cover crop, so several years ago I grew a cover crop and tested the plants. In early August, I planted a blend of Italian ryegrass, hairy vetch and buckwheat. Two months later, just before the first killing frost that fall, I tested stems, leaves and roots to determine what nutrients the cover crop had extracted. Based on my calculation of the number of pounds of biomass per acre produced by both the aboveground and belowground portions of the cover crop, the following table shows how many nutrients per acre were held in those plants.
As you can see, the cover crop pulled a lot of nutrients out of the soil and fixed some nitrogen from the air. Those nutrients will be held in the dead plant tissues and roots over the winter, and in the spring when microorganisms break down the tissues, most of those nutrients will be released back into the soil. In addition, the vetch and ryegrass will grow back in the spring and pull even more nutrients out of the soil. After I work the cover crop into the ground in the spring, those nutrients, along with carbon the plant fixed through photosynthesis, will become food for microbes and then will be released into the soil.
Not all plants pull the same nutrients out of the soil. Different types of plants take up varying amounts and types of nutrients. Corn and potatoes, for example, tend to need a lot of available minerals in the soil in order to get the nutrients they need. Small grains, on the other hand, can grow on low fertility soils. Oats and buckwheat in particular can grow and produce a crop on poor soils where other plants would struggle.
About the Author:
Raised on a Wisconsin dairy farm, Gary Zimmer is a world-renowned farmer, author, speaker, and biological farming consultant with over 35 years’ experience in agriculture. He has helped thousands of farmers to improve their operations, starting with the soil and building to a profitable, successful farming operation. Gary is the founder of Midwestern BioAg, and runs Otter Creek Organic Farm together with his family.